JPH11352089A - Liquid concentration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid concn. sensor which is not affected by the electrical conductivity of a liquid and ionic impurities in the liquid. SOLUTION: At least two conductive electrodes 12 supported by an electrode support plate 13 are immersed in a mixed liquid 14 of a liquid hydrocarbon and alcohol are insulated by an electrically insulating substance. The mixed liquid which is an object to be measured in concn. is introduced into a gap between two conductive electrodes. The conductive electrodes are provided to so as to hold the electrode plate interval by an electrode interval support material. The conductive electrode is connected to a connection terminal 18 by a conductor 17, and the convection terminal is connected to an integrated circuit by a lead wire. The measuring part is fixed to a tank, into which a liquid to be measured enters by a flange 15. The measuring circuit part is constituted of a rectangular wave generating circuit, an integrated circuit, a full-wave rectifying circuit, a smoothing circuit and an amplifying circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid concentration sensor for measuring the methanol content of a methanol-mixed fuel supplied to, for example, a vehicle equipped with a fuel cell from the dielectric constant of the fuel.

[0002]

2. Description of the Related Art FIG. 10 is a block diagram of a conventional dielectric constant measuring apparatus 1. This figure shows the sensor body 2 for measuring the concentration of liquid.
And a measurement circuit section 9. As the liquid, a methanol aqueous solution 3 in which methanol is mixed with water is used.

In the sensor body 2, the metal electrode 4 is directly immersed in the aqueous methanol solution 3 for use. Reference numeral 5 in the figure is a casing, which is formed of an insulating material. A coil 6 is wound around the outer periphery of the casing 5.

[0006] The measurement circuit section 9 is connected to the lead of the coil 6 and is connected to a parallel resistor forming a parallel circuit with the metal electrode 4. Further, the measured voltage is amplified by the amplifier,
A DC output voltage that is synchronously rectified and separated into a component that is in-phase and a component that is perpendicular to the measuring oscillator is input to an integrator. While the DC output voltage remains at the inputs of these integrators, the integrator output changes and the in-phase and quadrature components operate completely independently to minimize the unbalanced voltage, The input becomes zero and equilibrates, and the capacitance of the metal electrode is measured.
As a document which discloses a liquid concentration sensor having such a configuration, JP-A-6-58897 can be mentioned.

In such a conventional sensor, since the metal electrode 4 is exposed, the sensor body 2 is affected by the conductivity and ionic impurities of the liquid, and a leakage current is generated. Therefore, a large error occurs in the measured value of the capacitance.

Japanese Patent Application Laid-Open No. 5-93703 discloses a technique for correcting the temperature when measuring the alcohol concentration in a liquid.

[0007]

A sensor currently used for measuring the concentration of alcohol or the like generates a leakage current even though no impurities are mixed in the liquid. This is because even if no ionic substance is mixed in due to the exposed electrodes, a leakage current is generated due to the electric conductivity of the liquid.

That is, it is difficult to accurately measure the capacitance of the liquid due to the occurrence of the leakage current, and it has been difficult to measure the concentration of the liquid.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a liquid concentration sensor capable of measuring the capacitance of a liquid without being affected by leakage current. Another object of the present invention is to provide an inexpensive liquid concentration sensor. Still another object of the present invention is to provide
An object of the present invention is to provide a sensor main body that can be easily attached to a liquid tank or a flow path to be measured.

[0010]

Means for Solving the Problems A liquid concentration sensor according to one aspect of the present invention is configured as follows. A pair of electrodes immersed in the liquid to be measured, means for electrically insulating the pair of electrodes and the liquid, an integrating circuit connecting the pair of electrodes to a feedback portion thereof, A liquid concentration sensor for measuring the concentration of a desired component of the mixed liquid, comprising: means for obtaining a concentration from an output.

[0011] According to the invention according to one aspect of the present invention configured as described above, since the electrodes are insulated from the liquid, no leakage current occurs. Similarly, this electrode is not affected by the ionic and electrical conductivity of the liquid. Therefore, disturbance caused by leakage current or the like hardly enters the physical quantity detected by the electrode, and high-precision measurement can be performed. Further, since an electric circuit for compensating for the effects of ionic conductivity and electric conductivity is not required, the measurement circuit can have a simple structure. This makes it possible to reduce the weight and cost of the measurement circuit unit, and thus to reduce the weight and cost of the liquid concentration sensor.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the above, since a pair of electrodes form a capacitor, the distance between them is substantially uniform over the entire surface of the electrodes. In the example, a plate-like electrode was used. In order to maintain their parallelism, an electrode spacing maintaining material and an electrode support plate are used.

The insulating means is not particularly limited as long as it can maintain the insulating property between the liquid and the electrode. Examples of the insulating means include plastic, ceramics, and metal oxide films. For example, when this sensor is used for a mixed liquid of water and methanol, it is desirable that the plastic used for insulation has excellent methanol resistance, low water absorption, and excellent dimensional stability. As plastics satisfying these conditions, modified polyphenylene ether, polybutylene terephthalate, polyethylene naphthalate, polyphenylene sulfide, polyether imide, polyether sulfone, polyether ether ketone, modified polysulfone, thermoplastic polyimide, polyamide imide, polybenzimidazole , Polyphthalamides, polyketones and liquid crystal polymers are preferred. Desirably, polyphenylene sulfide, polyetherimide, polyethersulfone and polyamideimide are used. Most plastics can be used if the methanol concentration is 50% or less.

As the ceramics used for insulation, all those having insulation properties such as alumina, silica, titania, magnesia, zirconia, boron nitride, nitrogen carbide, silicon carbide, titanium carbide and diamond can be used. Desirably, a material having a relative dielectric constant close to that of the liquid to be measured and easy to form a thin film is preferable. When an oxide film or the like is formed on a metal plate, the film can be used as an insulating film.

The conductive electrode and the insulator need not necessarily be in contact with each other, and may have a gap. The insulator may cover at least a portion of the electrode that is immersed in the liquid.

In order to produce an electrode insulated by an insulator, a thermocompression bonding method, a coating method, a bonding method, an anodic oxidation method, or the like can be used. It is also possible to insert an insulator between the electrodes to be measured. The thermocompression bonding method is a method of joining by applying pressure while applying heat to the electrode and the insulator, by a method of using a hot press or a method of applying internal pressure by applying a current to the object to generate internal heat and join. Make it. Next, the coating method is to apply the insulator uniformly to the electrode surface using a spray, to disperse the insulator uniformly to the electrode surface using static electricity, to apply the insulator to the surface using a brush, and to reduce the pressure. Under the insulator, the physical vapor deposition (PVD), the chemical vapor deposition (CV
D) or by a method of uniformly applying by sputtering. The bonding method is a method in which an adhesive is thinly and uniformly applied to the surface of an electrode to bond an insulator.

As for the distance between the electrode plates of the sensor body, the smaller the distance than the basic formula of the capacitor, the larger the capacitance. However, if it is too narrow, the air bubbles between the electrode plates will not escape or the liquid to be measured will not easily enter. In an actual experiment, in the case of a methanol aqueous solution, when the distance between the electrode plates was 2 mm and the electrode area was 10 cm2, the capacitance was 100 pF or more.
The result was 0 pF or less. In addition, when increasing the electrode plate interval, the electrode plate area may be increased as shown in FIG.

A capacitor is formed by such an electrode and the liquid, and the concentration of the desired component of the liquid, for example, the methanol component in an aqueous methanol solution is specified by measuring the capacitance of the capacitor. For example, in the measuring circuit unit 30 of the embodiment shown in FIG. 5, the triangular wave S
The amplitude of 2 is inversely proportional to the capacitance C1 of the capacitor formed by the insulating electrode-liquid-insulating electrode. The relationship between the capacity of the condenser and the concentration of methanol is as shown in FIG. Therefore, the capacity of the capacitor is specified from the amplitude of the triangular wave, and the methanol concentration is specified from the capacity based on the relationship in FIG.

As is clear from FIG. 7, since the relationship between the capacitance and the concentration changes depending on the temperature, it is preferable to measure the temperature and correct the temperature.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a sensor main body 11 of a liquid concentration sensor 10 according to the embodiment. A pair of electrodes 12 and 12 made of a flat plate (25 × 40 mm) are embedded in electrode support plates 13 and 13 made of polyphenylene sulfide, respectively. Therefore, each electrode 12, 12 is electrically insulated from the aqueous methanol solution 14 by the material of the electrode support plates 13, 13. The electrode support plates 13, 13 are fixed vertically to the flange 15 (attachment means), and the distance (2 mm) between the electrodes 12, 12 is maintained at equal intervals over the entire surface. Further, the distance between the electrodes 12, 12 is defined by the electrode spacing maintaining member 16. The flange is fixed near the inlet of the aqueous methanol solution tank, so that the electrodes 12, 12 are immersed in the aqueous methanol solution.

According to the sensor main body 11 having such a configuration, the insulating electrodes 12 and 12 may be immersed in an aqueous methanol solution while maintaining a predetermined interval. Therefore, the mounting position is not limited to the vicinity of the tank inlet, but can be mounted at an arbitrary position in the tank or the flow path. On the other hand, in the conventional sensor main body 2, the coil 6 must be wound around the casing 5, so that the mounting position thereof is greatly restricted.

Reference numerals 17 and 18 in the figure denote a conductor and a connection terminal, respectively. Reference numeral 19 in the drawing indicates a temperature sensor.

FIG. 2 is a schematic view of a capacitor constituted by the sensor body 11 having such a configuration. The capacitance of the capacitor in this case is represented by the following equation. C = ε0ε1ε2S / ｛k (ε1−ε2) + ε2｝ d Equation (1) where ε0: dielectric constant of vacuum, ε1: relative dielectric constant of aqueous methanol solution, ε2: relative dielectric constant of insulating film, S: electrode area ,
d: distance between electrodes, k: ratio of insulating film thickness to distance between electrodes (t
(Insulating film thickness) = kd).

FIG. 3 shows the relationship between the capacitance and the methanol concentration when the thickness t of the insulating film is changed. The vertical axis shows the ratio between the capacitance C0 when there is no insulating film and the capacitance Cins when using the insulating film. Note that the relative dielectric constant ε2 of the insulating film was set to 2. FIG. 3 shows that the slope of the curve representing the relationship between the capacitance and the concentration increases as the thickness of the insulating film decreases. Therefore, from the viewpoint of reducing the error in specifying the concentration of methanol in the methanol aqueous solution, it is preferable that the thickness of the insulating film be as small as possible. In consideration of the durability of the insulating film, the ease of film formation, and the like, each of the electrodes 12, 1
It is preferable that the thickness of the insulating film (the portion indicated by the reference numeral 13a in FIG. 1) that covers the layer 2 is 0.01 μm to 1 mm. More preferably, the thickness is 0.01 μm to 2 mm.

FIG. 4 is a schematic diagram showing a sensor body according to another embodiment. In this sensor body, the electrode plates 21 and 21 are arranged on the periphery of a relatively large outlet port of the tank 25 for the mixed liquid 24. Note that the tank 25 for the mixed liquid 24 is entirely formed of an insulating synthetic resin, so that the electrode plates 21 are electrically insulated from the mixed liquid 24. A terminal is connected to each of the electrode plates 21 and 21 via a conductive wire (not shown).

FIG. 5 shows a measuring circuit section 30 according to the embodiment. The square wave S1 as a pulse signal generated by the square wave generation circuit 31 is input to the inverting input terminal of the integration circuit 33. In the integration circuit 33, the terminals 18, 18 of the sensor main body 11 are connected to the feedback section thereof in parallel with the resistor R2. As a result, the capacitor formed by the insulating electrode, the aqueous methanol solution, and the insulating electrode is connected to the feedback section of the integrating circuit. The output waveform of the integration circuit 33 becomes a triangular wave S2 as shown in the figure. The amplitude of the triangular wave S2 is inversely proportional to the capacitance C1 of the capacitor.

The triangular wave output of the integration circuit 33 is full-wave rectified by the full-wave rectification circuit 35, and its minus component is converted to a plus component. The peak voltage signal obtained by the full-wave rectifier circuit 35 is further led to a smoothing circuit 36 to be a DC voltage signal corresponding to the peak voltage signal. Then, this DC voltage signal is amplified by the amplifier circuit 37 to obtain a voltage signal S3 usable by the concentration specifying device 40.

FIG. 6 shows the concentration specifying device 40. The signal S3 from the amplifier circuit 37 is processed by the CPU 43 via the input interface 41. As described above, since the amplitude of the triangular wave S2 is inversely proportional to the capacitance C1 of the capacitor, the voltage signal S3 (specifically, １ ／ of the amplitude of the triangular wave S2) corresponding to the amplitude of the triangular wave S2 is electrostatic. It is inversely proportional to the capacitance C1. In other words, the reciprocal of the capacitance (1 / C1) is a function of the voltage signal S3. On the other hand, there is a specific relationship shown in FIG. 7 between the capacitance C1 and the concentration of the methanol component. Thus, the concentration of methanol is a function of the voltage signal S3. FIG. 7 shows that the relationship between the capacitance C1 and the concentration of the methanol component changes depending on the temperature. Therefore, in the concentration specifying device 40, the relationship between the voltage signal S3 and the concentration of the methanol component is stored in the memory 45 in advance. The storage method is not particularly limited, but in this embodiment, the storage method is a table format. Note that the relationship between the voltage signal S3 and the concentration is stored for each temperature of 0 ° C, 20 ° C, 40 ° C, and 60 ° C. Then, based on the voltage signal S3 and the temperature of the methanol aqueous solution detected by the temperature sensor 19, the CPU refers to the table stored in the memory 45 and specifies the concentration of the methanol component. If the temperature of the aqueous methanol solution is different from the temperature stored in the memory in advance, the data in the table is corrected by a known method such as linear interpolation.

FIG. 8 shows an example of the relationship between the voltage signal S3 (vertical axis) measured by the sensor of the embodiment and the methanol concentration (horizontal axis). The temperature of the methanol aqueous solution at the time when the result of FIG. 8 was obtained was 20 ° C. The concentration of the methanol component specified in this manner is output to the display device 50 via the output interface 47. For the display device 50, a general-purpose pointer-type meter or bar graph device is used.

FIG. 9 shows a comparison between the theoretical value (solid square) and the experimental value (○) of the capacitor formed by the insulating electrode, the aqueous methanol solution, and the insulating electrode, which are obtained from Equation 1. However, when the experimental value was obtained, the temperature of the methanol aqueous solution was 20 ° C., and the resistance value R1 was 10 K ohm. From the results in FIG. 9, it can be seen that the results obtained with the sensor of the example are very close to the theoretical values.

[0031]

As described above, in the liquid concentration sensor according to the present invention, since the conductive electrode is insulated by the insulator, it is not affected by the conductive and ionic conductive solutions, and at least one liquid in the liquid can be accurately measured. It has become possible to measure the concentration of various liquids.

Further, by using the pulse voltage of the measurement signal, the measurement circuit becomes simple and can be manufactured at a very low cost as compared with the case where an alternating current is used.

Therefore, even if a large number of sensors are manufactured and there are variations, it is easy to modify the sensors because of their simple structure. For this reason, it is possible to provide a device that always accurately measures the capacitance of the liquid regardless of the variation between the sensors.

The present invention is not at all limited to the description of the above-described embodiments and examples. Various modifications are included in the present invention without departing from the scope of the claims and within the scope of those skilled in the art.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a sensor main body of a liquid concentration sensor according to one embodiment of the present invention.

FIG. 2 is a conceptual diagram of a capacitor composed of an insulated electrode and a mixed liquid of the sensor body of the embodiment.

FIG. 3 is a graph showing the dependence of the concentration-capacitance curve of the capacitor of FIG. 2 on the thickness of an insulating film.

FIG. 4 is a conceptual diagram showing a sensor main body according to another embodiment.

FIG. 5 is a diagram illustrating a structure of a measurement circuit unit according to one embodiment.

FIG. 6 is a block diagram showing a configuration of a concentration specifying device.

FIG. 7 is a graph showing the temperature dependence of a concentration-capacitance (reciprocal) curve.

FIG. 8 is a graph showing the relationship between the voltage output S3 of the measurement circuit section of the embodiment and the methanol concentration.

FIG. 9 is a graph showing a comparison between a theoretical value obtained from Equation 1 and an experimental value of a capacitor formed of an insulating electrode, an aqueous methanol solution, and an insulating electrode.

FIG. 10 is a diagram showing a configuration of a conventional liquid concentration sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid concentration sensor 2, 11 Sensor main body 3, 14 Methanol aqueous solution 4, 6, 12, 21 Electrode 9, 30 Measuring circuit part 13a Insulating film 33 Integration circuit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsuhiko Saguchi 2-3-6 Shimonopporo Techno Park, Atsubetsu-ku, Sapporo, Hokkaido Inside Imura Japan Co., Ltd. (72) Masafumi Kobayashi 2-3-6, Shimonopporo Techno Park, Atsubetsu-ku, Sapporo, Hokkaido No. Within Imura Japan Co., Ltd. (72) Inventor Atsushi Ogino 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Yoshio Kimura 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation

Claims (14)

[Claims] 1. A liquid comprising at least two types of liquids.
In a liquid concentration sensor that measures the concentration of at least one type of liquid, (a) at least two conductive electrodes, and (b) the at least two electrodes are electrically insulated from the liquid by an insulator. A liquid concentration sensor. 2. The method according to claim 1, wherein a concentration of the at least one liquid is measured from a change in capacitance of a capacitor formed by the electrode and the liquid existing between the electrodes. Liquid concentration sensor. 3. The liquid concentration sensor according to claim 2, wherein the capacitance change is inserted into a feedback section of an integration circuit. 4. A measuring unit comprising: a pair of electrodes whose peripheral surfaces are insulated; means for maintaining a distance between the pair of electrodes; and a terminal for connecting the pair of electrodes to an external circuit. And a mounting means for attaching the measuring section to a tank of a mixed liquid to be measured or a flow path thereof, and a sensor main body of the liquid concentration sensor. 5. The sensor body according to claim 4, wherein said pair of electrodes are plate-shaped and covered with an insulator. 6. The sensor body according to claim 4, further comprising a temperature sensor. 7. A measuring circuit for use in a liquid concentration sensor in which a pair of insulating electrodes is immersed in a liquid to be measured, wherein the pair of insulating electrodes are connected to a feedback section thereof; And a means for obtaining a concentration from the output of the measurement circuit. 8. The measurement circuit according to claim 7, further comprising a circuit for inputting a DC pulse signal to said integration circuit. 9. The measuring circuit according to claim 8, wherein the DC pulse signal is a rectangular wave signal. 10. A pair of electrodes immersed in a liquid to be measured, means for electrically insulating the pair of electrodes from the liquid, and an integrating circuit connecting the pair of electrodes to a feedback portion thereof. A liquid concentration sensor for measuring the concentration of a desired component of the mixed liquid, comprising: means for obtaining a concentration from an output of the integration circuit. 11. The apparatus according to claim 1, further comprising means for applying a DC pulse signal to said integration circuit.
The liquid concentration sensor according to 0. 12. The liquid concentration sensor according to claim 11, wherein the DC pulse signal is a rectangular wave signal. 13. A capacitor section formed in a liquid to be measured, an integration circuit having the capacitor section connected to its feedback section, a circuit for inputting a signal to the integration circuit, and an output of the integration circuit. Means for converting the concentration of the liquid into a desired component based on a predetermined rule. 14. The apparatus according to claim 10, further comprising: a temperature sensor for measuring a temperature of the liquid; and means for correcting the predetermined rule based on a measurement result of the temperature sensor. A liquid concentration sensor according to any one of the above.